Epoxy−Silica Mesocomposites with Enhanced Tensile Properties and Oxygen Permeability
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Abstract
As-made and calcined forms of large-pore (5.3 nm) mesostructured silica with a wormhole framework structure, denoted MSU-J, have been used to form rubbery epoxy mesocomposites containing 1.0−12% (w/w) silica. The tensile modulus, strength, toughness, and extension-at-break for the mesocomposites formed from as-made and calcined forms of MSU-J silica are systematically reinforced by up to 4.8, 5.7, 1.6, and 8.5 times, respectively, in comparison to the pure epoxy polymer. The composites represent the first examples wherein the reinforcement benefits provided by mesostructured silica particles are comparable to those provided by exfoliated organoclay nanolayers at equivalent loadings. Moreover, the reinforcement benefits are realized without the need for organic modification of the silica surface, and the increases in tensile properties occur with little or no sacrifice in optical transparency or thermal stability. The oxygen permeability of the mesocomposites prepared from as-made MSU-J silica increases dramatically at loadings ≥5.0% (w/w), whereas the compositions made from the calcined form of the mesostructure show no permeation dependence on silica loading. For instance, the oxygen permeability of the mesocomposites containing 12% (w/w) as-made MSU-J silica is 6-fold higher than that of the silica-free epoxy membrane. Positron annihilation lifetime spectroscopy established the absence of free volume in the mesocomposites, thus precluding the possibility of facile oxygen diffusion through the framework pores of the silica. The increase in oxygen permeability is correlated with the partitioning of curing agent between the as-made mesostructure and the liquid prepolymer, which leads to coronas of permeable polymer with reduced chain cross-linking in the vicinity of the silica particles. Mesocomposites made from calcined forms of the mesostructured silica do not allow for curing agent partitioning, and the oxygen permeability is not significantly influenced by the silica loading.
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